Hitherto, a PS plate having a construction such that a lipophilic photosensitive resin layer is provided on a hydrophilic support has been broadly used as a lithographic printing plate precursor. As for the plate-making method thereof, the PS plate is ordinarily subjected to mask exposure (open-frame-exposure) through a lith film and then dissolving and removing the non-image area to obtain a desired printing plate.
In recent years, digitization techniques of electronically processing, accumulating and outputting image information using a computer have been widespread and various new image output systems corresponding thereto have been put into practical use. As a result, a computer-to-plate (CTP) technique of directly producing a printing plate without using a lith film but by scanning highly directional light such as laser light based on the digitized image information is demanded and it is now an important technical subject to obtain a printing plate precursor suitable for such a technique.
As one system for obtaining such a lithographic printing plate precursor capable of subjecting scanning exposure, a construction such that a photopolymerizable composition having excellent photosensitive speed is used as an ink-receptive photosensitive resin layer (hereinafter referred to as a “photosensitive layer”) provided on a hydrophilic support has been heretofore proposed and lithographic printing plate precursors having such a construction are already available on the market. The printing plate precursor having such a construction is subjected to development processing in a simple and easy way and exhibits desired plate-making and printing performances of providing excellent resolution, adhesion of ink, press life and scum resistance.
The photopolymerizable composition fundamentally comprises an ethylenically unsaturated compound, a photopolymerization initiation system and a binder resin, and the photo-initiation system absorbs light to produce an active radical and the radical induces the addition polymerization of the ethylenically unsaturated compound, as a result, the photosensitive layer is insolubilized, thereby effecting image formation.
Most of conventional proposals regarding the photopolymerizable composition capable of being subjected to scanning exposure utilize a photo-initiation system excellent in photosensitivity, and many compositions therefor are described, for example, in Non-patent Documents 1 and 2 shown below.
In conventional CTP systems using a photopolymerizable composition comprising the above-described initiation system and a long wavelength visible light source such as an Ar laser (488 nm) or FD-YAG laser (532 nm) as a light source, it is required to write at a higher speed in order to increase productivity in the plate-making step. However, such a requirement has not been achieved because the output of light source is not sufficiently high and the sensitivity of photosensitive material is not sufficiently high.
On the other hand, a semiconductor laser capable of performing continuous oscillation in the region of from 350 to 450 nm using, for example, an InGaN series material has been recently put into practical use. A scanning exposure system using such a short wavelength light source is advantageous in that an economical system can be constructed while maintaining sufficiently high output since the semiconductor laser can be produced at a low cost in view of its structure. Also, as compared with conventional systems using an FD-YAG or Ar laser, a photosensitive material having a photosensitive range in a short wavelength region and enabling working under brighter safe light can be used.
However, a photo-initiation system having a high sensitivity sufficiently for scanning exposure in the short wavelength region of from 350 to 450 nm has not yet been known.
In the imaging field widely, the technique for obtaining a photo-initiation system having a high sensitivity is still keenly demanded (see, for example, Patent Document 1 and Non-patent Documents 3 and 4 shown below). Such a photo-initiation system is also expected to application to image formation, for example, optical modeling, holography and color hard copy, to a field of production of an electronic material, for example, photoresist, or to use as a photocurable resin material, for example, ink, paint or adhesive. It has been desired in the fields of these industries to find a sensitizing dye excellent in light-absorbing property and sensitizing property in order to effectively induce decomposition of an active agent.    Non-patent Document 1: Bruce M. Monroe et al., Chemical Review, Vo. 93, pages 435 to 448 (1993)    Non-patent Document 2: R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, Vol. 73, pages 81 to 96 (1993)    Non-patent Document 3: J. P. Faussier, Photoinitiated Polymerization-Theory and Applications: Rapra Review, Vol. 9, Report, Rapra Technology (1998)    Non-patent Document 4: M. Tsunooka et al, Prog. Polym. Sci., Vol. 21, page 1 (1996).    Patent Document 1: JP-A-2000-258910 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)